Re: [PATCH] xfs: handle array index overrun in xfs_dir2_leaf_readbuf()

Brian Foster
2017-04-17T20:57:22Z

On Thu, Apr 13, 2017 at 01:45:43PM -0500, Eric Sandeen wrote:
> Carlos had a case where "find" seemed to start spinning
> forever and never return.
>
> This was on a filesystem with non-default multi-fsb (8k)
> directory blocks, and a fragmented directory with extents
> like this:
>
> 0:[0,133646,2,0]
> 1:[2,195888,1,0]
> 2:[3,195890,1,0]
> 3:[4,195892,1,0]
> 4:[5,195894,1,0]
> 5:[6,195896,1,0]
> 6:[7,195898,1,0]
> 7:[8,195900,1,0]
> 8:[9,195902,1,0]
> 9:[10,195908,1,0]
> 10:[11,195910,1,0]
> 11:[12,195912,1,0]
> 12:[13,195914,1,0]
> ...
>

This fix seems fine to me, but I'm wondering if this code may have
issues with other kinds of misalignment between the directory blocks and
underlying bmap extents as well. For example, what happens if we end up
with something like the following on an 8k dir fsb fs?

0:[0,xxx,3,0]
1:[3,xxx,1,0]

... or ...

0:[0,xxx,3,0]
1:[3,xxx,3,0]
...
N:[...]

Am I following correctly that we may end up assuming the wrong mapping
for the second dir fsb and/or possibly skipping blocks?

Brian

> i.e. the first extent is a contiguous 2-fsb dir block, but
> after that it is fragmented into 1 block extents.
>
> At the top of the readdir path, we allocate a mapping array
> which (for this filesystem geometry) can hold 10 extents; see
> the assignment to map_info->map_size. During readdir, we are
> therefore able to map extents 0 through 9 above into the array
> for readahead purposes. If we count by 2, we see that the last
> mapped index (9) is the first block of a 2-fsb directory block.
>
> At the end of xfs_dir2_leaf_readbuf() we have 2 loops to fill
> more readahead; the outer loop assumes one full dir block is
> processed each loop iteration, and an inner loop that ensures
> that this is so by advancing to the next extent until a full
> directory block is mapped.
>
> The problem is that this inner loop may step past the last
> extent in the mapping array as it tries to reach the end of
> the directory block. This will read garbage for the extent
> length, and as a result the loop control variable 'j' may
> become corrupted and never fail the loop conditional.
>
> The number of valid mappings we have in our array is stored
> in map->map_valid, so stop this inner loop based on that limit.
>
> There is an ASSERT at the top of the outer loop for this
> same condition, but we never made it out of the inner loop,
> so the ASSERT never fired.
>
> Huge appreciation for Carlos for debugging and isolating
> the problem.
>
> Debugged-and-analyzed-by: Carlos Maiolino <cmaiolin@redhat.com>
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
>
>
> diff --git a/fs/xfs/xfs_dir2_readdir.c b/fs/xfs/xfs_dir2_readdir.c
> index ad9396e..c45de72 100644
> --- a/fs/xfs/xfs_dir2_readdir.c
> +++ b/fs/xfs/xfs_dir2_readdir.c
> @@ -394,6 +394,7 @@ struct xfs_dir2_leaf_map_info {
>
> /*
> * Do we need more readahead?
> + * Each loop tries to process 1 full dir blk; last may be partial.
> */
> blk_start_plug(&plug);
> for (mip->ra_index = mip->ra_offset = i = 0;
> @@ -425,9 +426,14 @@ struct xfs_dir2_leaf_map_info {
> }
>
> /*
> - * Advance offset through the mapping table.
> + * Advance offset through the mapping table, processing a full
> + * dir block even if it is fragmented into several extents.
> + * But stop if we have consumed all valid mappings, even if
> + * it's not yet a full directory block.
> */
> - for (j = 0; j < geo->fsbcount; j += length ) {
> + for (j = 0;
> + j < geo->fsbcount && mip->ra_index < mip->map_valid;
> + j += length ) {
> /*
> * The rest of this extent but not more than a dir
> * block.